feat: config-based FFN activation + relu2 support for BitNet (fixes #602)

[Pranesh Nikhar (praneshnikhar)]

BitNet b1.58 uses squared ReLU (relu²) in the FFN, but the graph builder was hardcoded to SiLU and the GGUF converters never wrote a hidden_activation key.

This PR makes the activation config-driven:

llama.cpp submodule changes

1. src/llama-model.cpp: Add "relu2" → LLM_FFN_RELU_SQR mapping
2. src/models/bitnet.cpp: Read <arch>.hidden_activation from GGUF, default to SiLU for backward compat, use hparams.llm_ffn_op in graph

Main repo changes

3. utils/convert-hf-to-gguf-bitnet.py: Write hidden_act("relu2") for BitNet models
4. utils/convert-ms-to-gguf-bitnet.py: Write hidden_act("relu2") for BitNet models

Behavior

• New GGUF files (converted with updated converters): FFN uses ReLU² automatically
• Existing GGUF files (no hidden_activation key): FFN uses SiLU (unchanged behavior, no regression)

Depends on:isHuangXin/llama.cpp#4

Closes#602

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[Pranesh Nikhar (praneshnikhar)]

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